article: One day my wife and I went shopping at  the shop. We took the car as we had a lot of things to buy because my brother and his family were going to spend the weekend with us. We stopped the car in front of the shop. An hour later we came back to the car with a lot of things. Then the trouble started. We could not open the car door.
  "Oh, dear," said my wife, "What are you going to do?"
  "Let's ask that policeman," I said. The policeman was very kind and glad to help us. A few minutes later he got the door open. Just at that moment an angry man came up and shouted, "What are you doing with my car?"
  We looked at the number of the car and our faces turned very red.
question: They didn't know it was the wrong car until they looked at   _  .
options: the colour of the car the owner of the car the things in the car the number of the car